feat: Sprint 3 — C FFI bindings, WASM compilation, Python example, SDK docs

- Create quicproquo-ffi crate with 7 extern "C" functions: connect,
  login, send, receive, disconnect, last_error, free_string
  (produces libquicproquo_ffi.so and .a)
- Feature-gate quicproquo-core for WASM: identity, hybrid_kem,
  safety_numbers, sealed_sender, app_message, padding, transcript
  all compile to wasm32-unknown-unknown
- Add Python ctypes example (examples/python/qpq_client.py) with
  QpqClient wrapper class and CLI
- Add SDK documentation: FFI reference, WASM guide, qpq-gen generators
- Update Dockerfile for quicproquo-ffi workspace member

docs/src/SUMMARY.md

@@ -21,6 +21,9 @@
 - [Certificate Lifecycle and CA-Signed TLS](getting-started/certificate-lifecycle.md)
 - [Docker Deployment](getting-started/docker.md)
 - [Bot SDK](getting-started/bot-sdk.md)
+- [C FFI Bindings](getting-started/ffi.md)
+- [WASM Integration](getting-started/wasm.md)
+- [Code Generators (qpq-gen)](getting-started/generators.md)
 - [Demo Walkthrough: Alice and Bob](getting-started/demo-walkthrough.md)
 
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